This invention relates generally to hot gas engines, and in particular to a novel heating system for supplying the heat energy input to a Stirling engine.
U.S. Pat. No. 4,055,952, Johansson et al, relates to a Heating Device For An External Combustion Engine. A working gas, such as helium, is pumped back and forth through a closed path between low temperature compression and high temperature expansion cylinders. This path includes a heater and a cooler where heat is introduced and rejected respectively, and the cooperative effect is the execution of a thermodynamic cycle resulting in the development of mechanical output power at a crankshaft connected to the pistons which operate within the cylinders.
The heater described in that patent comprises a number of arcuately shaped tubes through which the working gas is conducted and a directly adjacent combustion device for heating the tubes. A liquid or gaseous fuel is combusted in a combustion chamber of the combustion device, and the hot gaseous products of combustion are caused to flow over the outside of the arcuately shaped tubes thereby heating the working gas which flows through the tubes. The engine of the patent possesses a number of characteristics which advantageously distinguish it from other power plants. They include multi-fuel use, relatively lower vibration and noise, and relatively smoother operation, among others.
The efficiency of the engine is related to the temperature of the working gas, the higher the temperature, the greater the efficiency. In a heater of the general type disclosed in the patent, the ability of materials to withstand elevated temperatures limits the temperature to which the working gas can be heated.
In actual construction, an engine like that shown in the patent comprises a brazed tube and fin assembly for maximizing the heat transfer surface area. Heat of the combustion gases is transferred through the fins to the tubes to heat the working gas. As in any heating device, the life of the heater is a function of the thermal stress to which the materials of its component parts are subjected. At higher. temperatures which are desirable for improving the engine efficiency, there are higher stresses and consequently a lower life expectancy. Accordingly, to avoid unacceptably low life expectancy, the heater is exposed to lower maximum temperatures, but this of course is at the expense of reduced engine efficiency. Maximum temperatures will occur at the fin tips, making the temperature tolerance of the fin material the limiting factor in improving the engine efficiency.
The present invention relates to a novel heating system for improving the efficiency of the engine while maintaining acceptable life expectancies for the heater. According to general principles of the invention, use of a condensing substance as the heat transfer fluid medium and the elimination of fins on the tubes can improve efficiency without compromising life expectancy as much as if temperature were raised in the prior heater designs shown in the patent and discussed above. With the present invention the outer surfaces of the tubes become the points of maximum temperature, rather than the fin tips, and the tube material therefore becomes the limiting factor in any trade-off between improved engine efficiency and acceptable heater life. Overall, a meaningful improvement is contemplated.
The invention still maintains one of the important advantages of this type of engine, namely the ability to be powered by different fuel sources. Other advantages inherent in the engine construction are also retained because the drive unit, namely the cylinders, block, and crankshaft, do not have to be modified. Of course the general principles of the invention are not limited to use in the specific engine configuration shown in the patent.
Briefly, the invention comprises what could be called a "heat pipe" type evaporator and condenser configuration forming a closed system for the condensing medium. Sodium is a suitable material for the medium. The sodium is heated in the evaporator and vaporized. It flows through a conduit to the condenser where it condenses onto the tubes which carry the hot working gas for the engine. The condensed liquid sodium flows back through the conduit into the evaporator where it is again vaporized. This is a continuous cycle whereby a thermal power flow from the evaporator to the condenser is continuously induced by the heating of the sodium in the evaporator.
Further aspects of the invention relate to the evaporator and the condenser details. The condenser comprises an outer cylindrical walled tube surrounding the tubes which carry the hot working gas. These latter tubes are arranged in a bundle, in which the individual tubes are parallel and of equal lengths, but spaced apart from each other within the bundle. Respective headers are at the respective ends of the bundle. The tubes have a lengthwise S-shape for thermal expansion and contraction, and the outer cylindrical walled tube contains an expansion joint for the same purpose.
The evaporator comprises inner and outer shells cooperatively arranged to form an evaporating chamber space of upright annular cup-shape. Returning condensate enters the evaporating chamber space at the chamber space's upper rim. It flows into an annular trough on the inner shell wall for annular distribution and subsequent overflowing of the trough to flow down the inner wall. A wicking material is applied to the wall to enhance the spreading of the condensate over the entirety of the wall for promoting efficient evaporation. A further shell is nested within the inner shell to cooperatively define an annular cup-shape heating passage for heating the inner shell, and conseqently heating the condensate, by heat flow through the inner shell wall. The heating passage is part of a combustion device which provides the heat; specifically, hot gaseous products of combustion flow through the annular cup-shape heating passage.
A single conduit communicates the evaporating chamber space with the condensing chamber space, and the condensate and the evaporate both flow through this conduit, but in opposite directions. Where the conduit enters the evaporator it is provided with one or more holes above the level of the returning condensate flow so that evaporate can pass from the evaporating chamber space into the conduit without blockage by the condensate.
The foregoing features, advantages, and benefits of the invention will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at this time in carrying out the invention.